404 



MOLLUSCA. 



The tribe includes three sections the Bulloidea, the Aplysioidea, and the 

 Pleurobrancliioidea. All the families formerly united as Gymnosomatous 

 Pteropoda are included with the Aplysioidea, while the thecosomatous Ptero- 

 pods are placed with the Bulloidea. All are marine. 



The leading features of the group are as follows: 



1. The displacement backwards of the pallial opening, the anus and the 

 circumanal complex. 



2. The disappearance of the visceral twist and of the operculum ; the reduc- 

 tion of the mantle and shell ; the exposure of the genital opening, and of 

 the ctenidium. 



3. The acquisition of external symmetry. 



4. The absence of the spermatic groove, which is transformed into a vas 

 deferens. 



5. The reduction in the number of teeth in the transverse rows of the radula. 



6. The concentration of the central nervous system, and the shortening of 

 the visceral commissure. 



7. The displacement of the oesophageal nerve collar towards the posterior 

 part of the buccal mass. 



In the following table the distribution of these characters amongst the families 

 is shown : 



Pleurobranchidae 

 Umbrellidae 

 Gymnosomata 

 Aplysiidae . 



Lobiger 



Pelta . 

 Thecosomata 

 Philinidae . 

 Bullidae 

 Actaeouidae 



* In some members of the type only. 



From this table it is apparent that Actaeon most resembles the Streptoneura, 

 inasmuch as it has its visceral commissure twisted, auricle anterior to ventricle, 

 an operculum, genital opening (female) in mantle-cavity, anus forward ; but 

 it differs from that group in being hermaphrodite. "With all these strepto- 

 neurous affinities one might expect the genital apparatus to be of the most 

 unspecialized construction, i.e. , with an hermaphrodite opening and perhaps 

 without even a spermatic groove, but this is not the case : in its ducts Actaeon 

 is as specialized as any Tectibranch. If we attempt to draw up a genealogical 

 scheme for the group, we cannot place Actaeon at the root because in this 

 particular it is as highly specialized as any form. Aplysia on the other hand, 



